Exhaust expelled from internal combustion engines such as automobile engines includes toxic components such as hydrocarbons (HC), carbon monoxide (CO), nitrogen oxides (NOx), etc. To efficiently eliminate these exhaust components, exhaust cleaning catalysts are used. An exhaust cleaning catalyst comprises a catalyst coating layer formed of a precious metal serving as the catalyst carried (supported) on a carrier or present as a solid solution with the carrier. As the precious metal, for instance, precious metals belonging to the platinum group (PGM) such as rhodium (Rh), palladium (Pd) and platinum (Pt) are used.
There are problems with these exhaust cleaning catalysts, such that their cleaning abilities decrease as the precious metal gradually degrades with the use. One of the main reasons for this is the sintering of the precious metals. In particular, upon exposure to exhaust at a high temperature (e.g. 800° C. to 1000° C.), the precious metal undergoes grain growth, resulting in a decrease in specific surface area (active spots) of the precious metal. As for the techniques to deal with such problems, for instance, Patent Documents 1 to 6 disclose preventing the degradation of precious metal by means of inhibiting the sintering of the “carrier.”